1. Field of the Invention
The present invention relates to a catheter for continuously measuring cardiac output by means of a cardiac catheter with a temperature sensing device.
2. Description of the Related Art
In order to measure cardiac output, the volumetric rate at which blood is pumped through the heart, the conventional practice is to employ an indicator dilution method. Thermo-dilution, which is one-of the indicator dilution methods available, involves injecting a cooled or heated solution directly into the flow of blood and subsequently measuring the temperature of the blood at some known distance downstream from the place where the solution is injected.
One disadvantage associated with the thermo-dilution method is that measurement of cardiac output is performed intermittently and not continuously. Moreover, the frequency of performing the procedure is limited because of the dilution of the patient's blood and the associated increase in fluid burden. In addition, the thermo-dilution method involves an increased risk of infection to the patient from contaminated injectate fluid or injectate syringes.
In an attempt to overcome these disadvantages, one device, described in U.S. Pat. No. 4,841,981, entitled "Catheters for Measurement of Cardiac Output and Blood Flow Velocity," measures the cardiac output according to the conventional thermo-dilution technique, and subsequently calculates blood flow velocity by means of a blood temperature sensing thermistor and a self-heating thermistor. Because the blood flow velocity multiplied by the cross-sectional area of the pulmonary artery is equal to the cardiac output, the cross-sectional area of the pulmonary artery may be determined once the cardiac output has been determined by means of the thermo-dilution method. After the thermo-dilution method has been performed once, subsequent cardiac output is determined using the velocity measurement multiplied by a constant obtained using the initial thermo-dilution measurement.
This method may provide continuous measurement of cardiac output provided that the cross sectional area of the pulmonary artery at the self-heated, measuring thermistor remains substantially constant. However, it has been observed that the parameter representing the blood vessel cross-sectional area, in fact, varies with time. Thus, in order to account for this variation, recalibration is carried out periodically by means of the thermo-dilution method in this device. In addition, the measuring location of the catheter may move into a region of the pulmonary artery having a different cross-sectional area. If the catheter so moves, the velocity calculation method provides inaccurate results until a new calibration is completed using thermo-dilution measurements. Finally, because calibration involves a conventional thermo-dilution cardiac output measurement, the risk of infection and fluid burden from increased fluid volume are still present.
Thus, although some attempts have been made to provide continuous monitoring of cardiac output, these previous devices are susceptible to inaccuracies in measuring cardiac output. Furthermore, these devices may place an undue cardiac burden upon the patient and increase the risk of infection.